The invention resides in an arrangement for the recirculation of exhaust gases in an internal combustion engine.
In order to achieve the lowest possible emission limits, particularly in connection with internal combustion engines for commercial vehicles, exhaust gas re-circulation concepts are being developed by which the NOx emissions are reduced without essential increases in the fuel consumption. It is known for example, from DE 43 30 487 C1 and DE 43 03 521 C1 to provide a turbocharger, for this purpose with an exhaust gas turbine which includes a variable geometry such as an adjustable flow guide structure, rotatable guide vanes, axially movable flow guide rings or similar devices. By changing the geometry of the exhaust gas turbine, for example, by closing the turbine flow guide structure, a turbine inlet pressures p.sub.3 which is larger than the charge air pressure P.sub.2 can be obtained over a large operating range up to full engine power. As a result, the exhaust gas can be re-circulated from a point of the exhaust pipe upstream of the turbine to the charge air duct downstream of a charge air cooler (inter-cooler) ahead of the engine without reducing the air/fuel ratio which would affect the fuel consumption.
There are however problems since during full load operation the cylinder pressure limit and also a compression pressure limit of the exhaust gas turbocharger must be observed, whereby exhaust gas re-circulation may be prevented or is at least limited. These limits can be overcome in known arrangements only by additional measures, which affect the overall efficiency of the gas exchange. Additional measures are disclosed, for example, in DE 4 429 232 C1, wherein variable ejectors serving as flow control apparatus, which are arranged in the charge air duct, are disclosed. Also, supplemental compressors for the compression of exhaust gas are known for example from DE 43 12 078 C2.
Using adjustable guide vanes, it is possible to form, during engine braking operation, a so-called turbo-brake wherein the turbine guide vanes structure is adjusted to a very small flow cross-section. As a result, the internal combustion engine is highly charged whereby a very high engine braking power can be obtained.
However, actual tests have shown that the long life expected from engines, and in particular from commercial internal combustion engines, cannot be achieved with the measures known so far.
DE OS 28 55 687 discloses an asymmetrical two-flow exhaust gas turbine, wherein the two spiral inlet flow passages, which are separated by a separating wall, are different in size. With the asymmetrical separation, it is said that the pressure ahead of the smaller inlet flow passage is higher than in the charge air duct so that exhaust gas re-circulation is made possible. In order to insure sufficient exhaust gas re-circulation also with high charge air pressures a large difference in flow cross-section between the two flow passages must be provided which detrimentally affects the turbine efficiency, and generally the charge air exchange efficiency of the engine.
It is the object of the present invention to provide an exhaust gas re-circulation arrangement for a supercharged internal combustion engine, which permits exhaust gas recirculation for the various operating states of the engine without expensive supplemental measures.